The invention described herein arose in the course of, or under, Contract No. DE-AC03-85SF16018 between the United States Department of Energy and Rockwell International Corporation.
This invention relates to an improved process for the production of high purity silicon nitride which includes the manufacture of a novel liquid solution consisting essentially of silicon, hydrogen, and nitrogen dissolved therein which may be further employed as an intermediate in a variety of chemical synthetic preparations, including usage as a precursor for the production of silicon nitride. More particularly, this invention relates to the production of high purity silicon nitride by the reaction of elemental silicon with a nitrogen-hydrogen reactant in its liquid state to produce high purity intermediate materials consisting of silicon, nitrogen, and hydrogen from which high purity silicon nitride may be formed by heating the intermediate materials.
Silicon nitride is an important structural material having outstanding potential for use in high strength applications, such as cutting tools for aerospace alloys, ball bearings, etc. The material is currently contemplated for use at high temperatures in turbine blades and ceramic diesel engines. The dielectric properties of this material also enable silicon nitride to be an important component in semiconductor barriers.
It would be desirable to have an industrial process for manufacturing high purity silicon nitride using elemental silicon, which is inexpensively available in high purity and which could be reacted at ambient temperature in a continuous automated process with a nitrogen-hydrogen reactant in its liquid state to form a high purity intermediate product consisting of silicon, nitrogen, and hydrogen from which high purity silicon nitride could be formed directly upon heat treatment without additional purification steps.
The original investigation of low and high temperature direct reactions between elemental silicon and nitrogen compounds was conducted by E. Vigouroux, as quoted by J.W. Mellor in A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VI, New York: Wiley, 1961, p. 163. He discovered that ammonia reacts with silicon at bright red heat, forming the nitride with liberation of hydrogen. High temperature nitridation of silicon is also detailed by Mangels U.S. Pat. No. 4,235,857 and is otherwise well known. However, ultra-high-purity silicon is extremely difficult to nitride at high temperature due to formation of protective nitride layers (exactly of the type used on semiconductors for passivation). According to S.S. Lin in an article entitled "Mass Spectrometric Studies on High Temperature Reaction Between Hydrogen Chloride and Silica/silicon" in the Journal Electrochem. Society, Vol. 123, 1976, pp. 512-514 and another article entitled "Comparative Studies of Metal Additives on the Nitridation of Silicon" in the Journal Am. Ceram. Soc., Vol. 60 (1-2), 1977, pp. 78-81; halide, iron, or other cation catalysts are required in such nitriding processes. D. Campos-Loriz et al, in an article entitled "The Effects of Hydrogen on the Nitridation of Silicon" in the Journal Mat. Sci., Vol. 14, 1979, pp. 1007-1008, and H. Dervisbegovic et al in an article entitled "The Role of Hydrogen in the Nitridation of Silicon Powder Compacts" in the Journal Mat. Sci, Vol. 16, 1979, pp. 1945-55, further explored the catalytic effects of hydrogen and water vapor on nitridation of silicon with a view to overcome the sluggishness and high expense of the process.
E. Vigouroux was cited in Mellor, p. 163, as unsuccessfully attempting to react silicon with liquid ammonia at low temperatures. These findings made it apparent to workers in the field that more reactive silicon derivatives, such as silicon chloride, silane, etc., would have to be employed to manufacture Si.sub.3 N.sub.4 at temperatures below 100.degree. C., the preferred temperature range for industrial processing.
Silicon nitride is, therefore, now industrially produced on the largest scale by the low temperature reaction of silicon tetrachloride with liquid ammonia as described in Iwai et al, U.S. Pat. No. 4,196,178. The amorphous silicon diimide intermediate may be crystallized to alpha silicon nitride upon heat treatment in a nitrogen atmosphere. However, the product formed tends to retain chloride on particle surfaces thereby not possessing the requisite purity needed for the above described desirable applications. Lengthy extractions with liquid ammonia or vacuum treatments to remove the halide from the final product incur additional processing costs. Oxygen contamination can result during this purification procedure due to the air and moisture sensitivity of the diimide intermediate. Even in the gas phase reaction between silicon tetrachloride and ammonia at temperatures over 700.degree. C., chlorine and oxygen contamination cannot reasonably be eliminated as described by M. Rahaman et al in an article entitled "Surface Characterization of Silicon Nitride and Silicon Carbide Powders" in the Am. Ceram. Soc. Bull., Vol. 65 (8), 1986, pp. 1171-76.
The use of silicon halides as the source of silicon for reaction with a nitrogen-containing material to form silicon nitride is well known, and such reactions have been described in a number of U.S. Pat. Nos., such as, for example, Mazdiyasni et al 3,959,446; Buljan et al 4,073,845; Mazdiyasni et al 4,113,830; Sussmuth 4,122,220; Mehalchick et al 4,145,224; Kleiner et al 4,208,215; Inoue et al 4,368,180; Buljan et al 4,376,652; and Sato et al 4,399,115.
Formation of silicon nitride using a silane as the source of silicon for reaction with a nitrogen compound has also been proposed. Prochazka et al in U.S. Pat. No. 4,122,155 and Kasai et al in U.S. Pat. Nos. 4,346,068; 4,387,079; and 4,612,297 teach the use of a silane as the source of silicon in such a reaction. Usage of silane (SiH.sub.4), a highly reactive gas at temperatures above -112.degree. C., can be problematic due to its explosive nature, lengthy product deposition times, inadequate control of product stoichiometry and morphology, and the overall costly economics of the process. Reactions with halogen-substituted silanes, as cited in the latter two patents, can add impurity problems, requiring extra purification costs.
Synthesis of silicon nitride can be achieved at elevated temperatures (from 1000.degree. C. to 1700.degree. C.) by carbothermic nitriding of silicon dioxide. This method is described in U.S. Pat. Nos. Mori et al 4,122,152; Komeya et al 4,428,916; and Hasimoto et al 4,590,053.
Processes for producing silicon nitride from mono- and di- sulfides of silicon with ammonia have been demonstrated at high temperature in Forsyth U.S. Pat. No. 3,211,527 and Morgan et al U.S. Pat. No. 4,552,740. These synthetic routes are more attractive than the above mentioned high temperature methods because high reactivity and varied morphologies can be produced by vapor transport and VLS mechanisms. In addition, sulfur volatilizes more cleanly from the product.
It would be desirable to have a process wherein elemental silicon, which is available as a high purity starting material, could be reacted at a temperature, equivalent at atmospheric pressure to below 200.degree. C., and preferably below 100.degree. C., with a nitrogen-hydrogen reactant in its liquid state to form a high purity intermediate product consisting essentially of silicon, nitrogen, and hydrogen from which high purity silicon nitride can be formed by a heat treatment.